Devices are known that are equipped with pacing and sensing circuits associated with the atrium and the ventricle, that can operate according to two known operating modes, DDD or AAI (the AAI mode being a DDD mode having a lengthened atrio-ventricular delay). These devices may be equipped with a mode called “DDD-AMC” or “AAISafeR” ensuring an automatic mode commutation from DDD to AAI and conversely.
The basic operating mode of a DDD/AAI pacemaker is the AAI mode—or more precisely a “pseudo-AAI” mode—with a single chamber atrial pacing (AAI mode stricto sensu), and a monitoring of ventricular activity. This operating mode is maintained as long as atrio-ventricular conduction is normal, that is, as long as each atrial event (either an atrial detection, corresponding to a spontaneous activity, or an atrial stimulation corresponding to a paced event) is followed by an associated ventricular detection.
In certain circumstances, however, atrio-ventricular blocks (“A-V blocks” or “AVB”) may appear, leading to a temporary disorder of depolarization of the ventricle. In this case, as long as several conditions are met, the pacemaker automatically commutes to automatic DDD mode, with parameters that are optimized for this situation of temporary AV block. After disappearance of the AVB, there is a reestablishment of atrio-ventricular conduction, and the pacemaker operation automatically commutes back to AAI mode, as long as several other conditions are met.
As it will be explained, a person of ordinary skill in the art should understand that the invention is not restricted to those devices with automatic mode commutation, but may also apply to devices operating in AAI mode only, insofar as the invention simply proposes to adapt the AAI mode operation without any automatic mode commutation to occur and/or so as to prevent automatic mode commutation towards DDD mode.
Such a pacemaker of AAI or DDD/AAI type is described in EP-A-1 470 836 and its counterpart U.S. Published Pat. Appl. No. 2005/0240235 (commonly assigned herewith to ELA Medical).
The present invention is based upon some observations that have been made while actually following up patients implanted with DDD/AAI pacemakers with automatic mode commutation features. It has been observed that such devices do not react in an optimal manner when certain types of ventricular tachycardiae (VT) appear that have a relatively stable rate that is close to twice the atrial pacing rate. Indeed, as it will be further explained below in the detailed description of FIG. 1, the “safety window” successive to each atrial pacing pulse is hiding one ventricular event out of two. As a result, the device senses a ventricular rate that is half of the actual rate. Therefore, aside from the fact that the ventricular rhythm associated with this tachycardia is not detected, the operation of the device is deceived, which may lead to a false diagnostic leading to an unexpected mode commutation towards DDD mode. Although a DDD mode operation usually has no deleterious effect to the patient, such a commutation is useless and hinders spontaneous atrio-ventricular conduction, which may prevent from diagnosing some other ventricular rhythm disorders for instance, and is in any event undesirable, if not needed.
Clinical observations also show that such a “non-expected” operation due to the hiding of some ventricular events may also occur when in the presence of an AV block, or when the AV cross-talk (hereinafter referred to as “AVCT”) condition is happening. This AVCT condition occurs when the device is sensing in the ventricular chamber a signal that is actually coming from a distant electrical stimulus in the atrial chamber. Such a condition needs to be properly detected and characterized so as to be able to diagnose the actual occurrence of an AV block.